Numerical Simulations of Multiphase Winds and Fountains from Star-forming Galactic Disks. I. Solar Neighborhood TIGRESS Model

Abstract

Gas blown away from galactic disks by supernova (SN) feedback plays a key role in galaxy evolution. We investigate outflows utilizing the solar neighborhood model of our high-resolution, local galactic disk simulation suite, TIGRESS. In our numerical implementation, star formation and SN feedback are self-consistently treated and well resolved in the multiphase, turbulent, magnetized interstellar medium. Bursts of star formation produce spatially and temporally correlated SNe that drive strong outflows, consisting of hot (T > 5 x 10(5) K) winds and warm (5050 K < T < 2 x 10(4) K) fountains. The hot gas at distance d > 1 kpc from the midplane has mass and energy fluxes nearly constant with d. The hot flow escapes our local Cartesian box barely affected by gravity, and is expected to accelerate up to terminal velocity of v(wind) similar to 350-500 km s(-1). The mean mass and energy loading factors of the hot wind are 0.1 and 0.02, respectively. For warm gas, the mean outward mass flux through d = 1 kpc is comparable to the mean star formation rate, but only a small fraction of this gas is at velocity > 50 km s-1. Thus, the warm outflows eventually fall back as inflows. The warm fountain flows are created by expanding hot superbubbles at d < 1 kpc; at larger d neither ram pressure acceleration nor cooling transfers significant momentum or energy flux from the hot wind to the warm outflow. The velocity distribution at launching near d similar to 1 kpc is a better representation of warm outflows than a single mass loading factor, potentially enabling development of subgrid models for warm galactic winds in arbitrary large-scale galactic potentials.